In recent years, epitaxial growth techniques, in which another crystal is grown on a crystalline substrate, such as a silicon substrate, to have a structure that corresponds to the crystal lattice of the substrate, have been indispensable in the processes for high performance devices such as heterobipolar transistors and elevated source/drain CMOS devices. If contamination exists on the topmost surface of the substrate when carrying out epitaxial growth, the contamination acts as seeds and crystal defects are generated in the epitaxially grown another crystal. Even if the density of the generated crystal defects is not very large, the contamination buried in the crystals adversely affects the characteristics of the device that uses the crystals. For these reasons, in order to produce a high performance device using an epitaxial growth technique, a cleaning in which contamination is removed from the surface of the crystal layer has to be performed prior to the start of the epitaxial growth process.
The cleaning is generally accomplished by means of a washing step, which is carried out outside the crystal growing apparatus, and a thermal cleaning, which is carried out inside the crystal growing apparatus. The washing step is carried out by treating the substrate with chemical solutions, such as acids and hydrogen peroxide, the purpose of which is to remove most of the contamination attached onto the substrate surface. Unlike other processes in the manufacturing steps for a semiconductor device, the cleanliness of a substrate surface that is required for the epitaxial growth process is very high. Thus, the purpose of the thermal cleaning carried out in the chamber is to remove the contamination that cannot be removed by the chemical washing step and to remove the oxide film formed due to reaction of the semiconductor, which composes the substrate with oxygen in the atmosphere subsequent to the chemical washing step or the contamination that has reattached to the substrate surface.
This thermal cleaning is carried out by heating the substrate under a specified atmosphere while the substrate is retained in a chamber. Also, a hydrogen atmosphere or a high vacuum atmosphere is used as an atmosphere in which the thermal cleaning can be carried out.
FIGS. 6(a), 6(b), and 6(c) are timing charts respectively showing the changes over time of wafer temperature, hydrogen flow rate, and open/close operation of an exhaust system during a conventional process including thermal cleaning of a substrate (wafer) under a low-pressure hydrogen atmosphere and epitaxial growth. In this atmosphere, the state of the atmosphere in the chamber is in a viscous flow region since the pressure in the chamber is relatively high, and the transport of gas is in a condition that can be represented in terms of fluid dynamics, in which interaction between the molecules is presumed. As shown in FIG. 6(a), for the cleaning under a hydrogen atmosphere, a thermal cleaning is performed under a pressure of several hundred to several thousand Pa (several Torrs to several ten Torrs) by heating a wafer to a wafer temperature Tc of between 850° C. and 900° C. and supplying hydrogen to the chamber at several liter/min. Thereafter, the temperature in the chamber is reduced to the temperature Te (about 550° C. in the case of epitaxial growth) for a subsequent treatment, and a treatment such as epitaxial growth is carried out. During that time, as shown in FIGS. 6(b) and 6(c), the exhaust system is kept open and a constant hydrogen flow rate Hfl is maintained. Thus, in the case of the thermal cleaning of a wafer under a hydrogen atmosphere, it is necessary that the wafer temperature be about 850° C. or higher in order to complete the thermal cleaning within a treatment time of several minutes.
FIGS. 7(a), 7(b), and 7(c) are timing charts respectively showing the changes over time of wafer temperature, hydrogen flow rate, and open/close operation of an exhaust system during a conventional process including thermal cleaning of a substrate (wafer) under a high vacuum atmosphere and epitaxial growth. As shown in FIG. 7(a), the thermal cleaning under a high vacuum atmosphere is usually carried out by heating the wafer to a wafer temperature Tc of between about 700° C. and 750° C. Thereafter, the temperature in the chamber is reduced to the temperature Te (about 550° C. in the case of epitaxial growth) for a subsequent treatment, and a treatment such as epitaxial growth is carried out. During that time, as shown in FIGS. 7(b) and 7(c), the high vacuum exhaust system is kept open and the hydrogen flow rate Hfl is maintained at 0, that is, hydrogen is not supplied. Thus, in the case of employing a high vacuum, generally no gas is introduced into the chamber during the thermal cleaning, but a small amount of several sccm per minute of disilane, hydrogen, or the like may be introduced. In this atmosphere, since the introduced amount of gas is small, the pressure range inside the chamber is 10−2 to 10−8 Pa (10−4 to 10−10 Torr), which is very low. In this high vacuum atmosphere, the gas transport is in a so-called molecular flow region, in which there is almost no interaction between molecules, and in this state there is also little interaction between gas molecules. In this case, the wafer temperature Tc may be about 550° C. (approximately the same as the growth temperature).
In the thermal cleaning method as described above, the types of atmospheres are selected according to the types of exhaust systems of the apparatus used for a treatment subsequent to the thermal cleaning, because the pressure range in the respective atmospheres varies greatly. Generally, thermal cleaning under a hydrogen atmosphere is used as a pretreatment for a low pressure chemical vapor deposition method (LP-CVD method), while the thermal cleaning under a high vacuum atmosphere is used as a pretreatment for an ultrahigh vacuum chemical vapor deposition method (UHV-CVD method) and for a molecular beam epitaxy method (MBE method) using a solid source.
FIGS. 8(a), 8(b), and 8(c) are timing charts respectively showing the changes over time of wafer temperature, hydrogen flow rate, and open/close operation of an exhaust system during a conventional process reported in reference (K. Oda et al., Journal of Electrochemical Society, Vol. 143, No. 7, p. 2361, 1966), which includes thermal cleaning under a high vacuum atmosphere and epitaxial growth. In this example, as shown in FIGS. 8(a) and 8(b), a thermal cleaning is performed under a pressure of several hundred to several thousand Pa (several Torrs to several ten Torrs) by heating a wafer to a wafer temperature Tc between 850° C. and 900° C. under a hydrogen atmosphere and supplying hydrogen into the chamber at several liter/min. Thereafter, an epitaxial growth process is performed under a high vacuum atmosphere. The apparatus is equipped with a high vacuum exhaust system and a low vacuum exhaust system, and as shown in FIG. 8(c), during the thermal cleaning, the high vacuum exhaust system is closed while the low vacuum exhaust system is opened, and during the epitaxial growth, the low vacuum exhaust system is closed while the high vacuum exhaust system is opened. In this method, a hydrogen atmosphere of a viscous flow region is used as an ambient within the chamber only during the thermal cleaning. However, in the case of this method, the condition of the atmosphere in the chamber is always in a viscous flow region from the start of the heating until the completion of the thermal cleaning, and after the thermal cleaning has completed, the inside of the chamber is changed over to an ultrahigh vacuum condition to start crystal growth. Thus, as a thermal cleaning method, this method is not different from the wafer cleaning treatment under a hydrogen atmosphere shown in FIG. 1.